The present invention relates to measurement and analysis of cable television (CATV) systems on a non-interfering basis, and more particularly to a CATV sweep system using a gated receiver for measuring the frequency characteristics of each channel of the CATV system in response to a radio frequency (RF) pulse inserted into the video signal during a spectrally dead period, the RF pulse having an amplitude that does not produce interference.
Broadband cable television (CATV) systems typically include active class-A broadband amplifiers and passive connectors, splitters and taps, all interconnected by a significant amount of coaxial cable. The majority of these components are in outside locations, exposed to temperature and weather extremes. Proper performance of such a CATV system is critical for customer satisfaction and continued regulatory compliance, and is affected by these extremes. Therefore the frequency response of the system, including all passive and active components, is important.
A common approach to determining frequency response is to inject a test signal at the headend of the CATV system. The test signal sweeps across the entire system bandwidth. Simultaneously the signal amplitude is measured at various points along the system to determine system gain and flatness. The difficulty with this approach lies not in actually performing the measurement, but rather in performing it while the system is operating without degrading the video signals being transmitted on the system. Historically high level, low level, intermediate level, and even "sweepless" sweep approaches have been tried. All these approaches suffer from various shortcomings including:
interference with cable signals; PA0 insufficient "sweep-to-noise" after several amplifiers; PA0 delayed response leading to "rubber screwdriver" effect; and PA0 too few data points across the system bandwidth.
One solution, incorporated in the 2721/2722 Non-Interfering Sweep System manufactured by Tektronix, Inc. of Beaverton, Oreg., United States of America, is to transmit short test pulses, approximately 8 microseconds in duration, during the vertical blanking interval of the video signals being carried by the CATV system. Since there is no video information transmitted during the vertical blanking interval, the picture quality theoretically is unaffected. The test pulse amplitude is set close to that of the system carriers, such as 6 dB down from the horizontal sync tip amplitude, so the pulses do not get lost in system noise. The measured amplitudes of these pulses are compiled to show the frequency response of the CATV system.
Once the measuring signal, which is generally an RF pulse, is inserted into the vertical interval, the question becomes one of determining an appropriate amplitude so the sweeper is truly non-interfering. If the amplitude of the RF pulse is set too large, it causes interference in the sound channel of the customer's set that sounds like ignition noise. On the other hand if the amplitude is set too small, the measured results have uncertainties caused by interference from the video signal in the channel being measured.
The mechanism of sound channel interference is fairly straight forward. When the customer's set receives a video signal together with the RF pulse, it treats the RF pulse as if it were part of the video signal. For example, if the RF pulse is 1.0 MHz higher in frequency than the channel picture carrier, then during the time that the RF pulse is "on" the receiver treats the video signal as if a 1.0 MHz sine wave were superimposed upon it. If the amplitude of the RF pulse with respect to the video signal is such that the sum of the two signals exceeds either the sync-tip or peak white values of the video signal, then the receiver's standards have been exceeded. When the standards are exceeded, some sets exhibit interference effects. These effects may be noticed as a buzz that accompanies video that exceeds peak white. In the case of very short pulses, such as the RF pulse, the buzz is reduced to a "pop." However since the RF pulse is transmitted while the video is at blanking level, it takes a much smaller pulse amplitude to exceed sync tip amplitude than it takes to exceed peak white.
The problem with sending a signal that exceeds sync tip amplitude is the compression and distortion it causes in the customer's receiver. There are no guarantees of how much extra amplitude over the sync tip level the receiver's IF output stage can handle. The receiver's AGC loop always adjusts the video signal in the IF stage to be at or near the maximum amplitude that this amplifier can handle. If the video signal plus the RF pulse causes the amplifier to compress, it causes the instantaneous amplitude of the 4.5 MHz IF sound intercarrier signal to decrease below the limiter's input threshold, causing a short term sound drop out, i.e., "pop", very similar in cause to the buzz caused by high video modulation. If the frequency of the pulse is such that the difference between it and the picture carrier is a sub-multiple of the 4.5 MHz sound inter-carrier spacing, the distortion caused by the RF pulse causing compression in the amplifier produces a harmonic that falls near the sound carrier, possibly also causing a pop.
Thus the RF pulse amplitude should be low enough so that receiver system standards are not violated. Assume that a TV modulator is transmitting blanking such as during the vertical interval between equalizing pulses. The amplitude of blanking is 75% of sync tip amplitude. Adding a 500 kHz sine wave signal to the video waveform and adjusting its amplitude so that the peak amplitude is just equal to 100% of sync amplitude, as shown in FIG. 1, results in each sideband of the sine wave being 12.5%. The total envelope amplitude of the video signal is the instantaneous sum of the three vectors shown. When the three vectors are aligned, they add to 100%. Thus each sideband is 18 dB below sync tip amplitude as opposed to the 6 dB of the 2721/2 Non-interfering Sweep System. This is the maximum amplitude that the RF pulse can be without exceeding receiver system standards.
To determine the amplitude to which the test pulse can be lowered, certain factors have to be considered. For example the test pulse must be larger than the specific amplitude of various portions of the video signal. The 2721/2 Non-interfering Sweep System places the RF pulse in a spectrally dead period between equalizing pulses, as indicated above. A VITS multiburst test signal generates a 100 IRE peak-to-peak sine wave similar to that caused by the RF pulse. An analysis similar to that of FIG. 1 shows that the multiburst signal causes a spectral component 16 dB below sync tip. The test pulse must be at least 2 dB above this amplitude so that an unequivocal peak measurement of the pulse can be made. Therefore the pulse amplitude can be set to a minimum of 14 dB below sync tip without allowing any tolerancing for amplitude variations in the CATV system. However this is 4 dB above the guaranteed non-interfering amplitude. Narrowing the receiver bandwidth also helps some, reducing spectral clutter by about 5 dB. But such a narrowing of bandwidth requires a longer pulse. Even with the lower bandwidth the RF pulse amplitude cannot be lowered below the minimum of 18 dB below sync tip to guarantee a non-interfering sweep. As indicated above the RF pulse amplitude generated by the Tektronix CATV Sweeper System is 6 dB below sync tip amplitude.
Another attempted solution to the problem of CATV system testing on a non-interfering basis is disclosed in U.S. Pat. No. 4,408,227 issued to Bradley on Oct. 4, 1983 entitled "Method and Apparatus for Television Distribution System Sweep Testing." The Bradley system uses time division multiplexing between the test signal and the video signal as a means of eliminating interference to the video signal due to testing. The video signal provides synchronizing signals that enable the video signal to be suppressed during the vertical interval and the test signal to be inserted in its place. At the receiver the process is reversed to demultiplex out the test signal for display. However, the Bradley system causes a loud buzz on the customer's receiver due to dropping the sound carrier when the test signal is multiplexed with the video signal. This is similar to the buzz caused by excess peak-white modulation mentioned above.
What is desired is a CATV sweep receiver for measuring test signals in the video signal that allow the amplitude of the pulse signal to be less than a threshold value that guarantees non-interference on a customer's receiver.